The following are some common optional features that some implementations may choose to support (though this list should not be taken to exclude additional extensions as well).
It is optional to support the specifying of residual properties (by name, not wildcard) in the SELECT, WHERE and ORDER BY clauses.
It is optional to support properties in the SELECT, WHERE and ORDER BY clauses that are not explicitly defined in the node types listed in the FROM clause but which are defined in subtypes of those node types.
It is optional to support the specifying of properties in the SELECT, WHERE and ORDER BY clauses that are not explicitly defined in the node types listed in the FROM clause but which are defined in mixin node types that may be assigned to node instances of the types that are mentioned in the SELECT clause.